Heretofore, interfitting timing gears have been provided on parallel rotating shafts to provide a precise angular relation between a drive shaft and a driven shaft as required for the operation of various devices, such as twin screw pumps, for example. A key on each shaft normally fits in an aligned elongate keyway in the internal bore of the associated timing gear for mounting the timing gear at a precise angular position on the shaft. Thus, the timing gears are easily mounted initially at a precise angular position on each shaft. However, after prolonged periods of use or undue wear, it is necessary to replace the timing gears, and the timing gears must be positioned on the shaft at a precise angular relation within a tolerance of about 0.001 inch for each 0.004 inch of circumference for twin screw pumps, for example.
For replacement of the timing gears, the worn timing gears are removed from the shafts upon longitudinal movement of the keyways in the gears relative to the keys on the shafts. Upon replacement of the timing gears, one of the timing gears is keyed to a shaft at a predetermined angular relation. The other timing gear for precise angular alignment has a keyway cut thereon at a precise angular relation on the shaft, but oftentimes the keyway is not cut at the precise location, thereby resulting in an improper timed position. Timing gears have been utilized on twin screw pumps which are single stage, positive displacement pumps used to transfer oil or other liquids of varying viscosities. The flow of liquid through the pump is accomplished by the progressive movement of sealed cavities formed by the intermeshing of matching pumping screws (one right hand and one left hand) rotating in the precision ground bores of the pump body. The key assembly of the screw pump is the rotating element. Each rotating element consists of a drive shaft and a driven shaft extending along parallel axes at a fixed center distance. Each shaft includes bearings, one timing gear and two opposing pumping screws plus mounting hardware. Some designs have pinned screws mounted on shafts, and others have the screws and shafts as an integral piece.
For proper operation, precise clearances are maintained between meshing screws to limit the internal leakage (slip) in the pump. The majority of two rotor, or two shaft pumps use timing gears. Timing gears are used to maintain these clearances, prevent contact between the pumping screws, and turn the driven shaft.
Timing gears currently in use are one part gears, and can be worm, spur, herringbone or helical gears, depending on the pump manufacturer. Replacement of these gears requires the complete removal of both rotating shaft assemblies from the pump, and requires the service of a precision machine shop to replace these gears. the timing gears are pressed into the shafts, and have keyways to prevent their rotation.
If new timing gears are to be installed on the rotating elements they must be timed before key-slotting. Spare gears are supplied in matched pairs with one key slotted and one not key-slotted. The reason for this is that the timing gear position on the shaft determines the critical clearance between pumping screws. In the average screw pump the change in clearance between the meshing screws is about 0.001 inch for each 0.004 inch distance in circumference at the pitch diameter of the timing gear. Since the normal axial clearance of meshing screws varies with the size of the pump and the viscosity of the fluid that the pump was designed for, very accurate key-slotting is essential for proper operation. The original screw clearances are stamped on the pump body at the bracket flange. This number represents total clearance. One half of it is the proper axial distance between the meshing screw threads.
For replacement, the old timing gears are removed from the shafts and the new gears are pushed half way onto their shaft diameter. The key is then placed in the slotted gear and the rotating elements are intermeshed. The free timing gear is adjusted until the desired screw clearance is obtained. The timing gears are then pushed into their final position and clearances are checked. If satisfactory, the rotating element is replaced in the body and the pump is reassembled without the stuffing box packing. The pump should now turn freely by hand. If not, check and repeat the above until proper results are obtained. When the results are satisfactory disassemble the pump. Check the screw clearances as before. If the clearances are correct, mark the timing gear from the key slot in the shaft. Then, cut the key slot in the gear with the greatest precision possible. Replace the gear with the key in place. Check the screw clearances as before. If satisfactory, assemble the pump and check for free rotation. If the key slot cut does not give the proper clearances, another key slot is cut to get proper results.
Unless the timing of screw pumps is done accurately, there is little point in doing it. However, periodic checks of the pump and replacement of timing gears as they wear are the best means of obtaining long service. Pump life may be extended three or more times by careful timing gear maintenance. If the timing gears are damaged suddenly (by foreign matter in the fluid being pumped) it is possible to effect temporary repairs by turning the gears on the shaft so the undamaged faces will be used.
It is an object of this invention to provide an apparatus and method for replacing interfitting timing gears on a pair of parallel shafts without the necessity of cutting a new keyway or key slot in one of the timing gears in order to provide a precise angular relation between the shafts.